Oscar Pistorius smiles during his trial at the high court in Pretoria on Monday. He is charged in the shooting death of his girlfriend, Reeva Steenkamp, in the predawn hours of Valentine's Day last year. | AP

The murder trial of Oscar Pistorius opened Monday in South Africa with testimony from a neighbor who described the sound of what she said were four gunshots and recalled the “blood-curdling screams” of a woman who prosecutors say was the girlfriend slain by the onetime star athlete in his home.

“It’s the most helpless feeling I’ve ever had in my life,” university lecturer Michelle Burger said of listening to the screams. “I knew something terrible was happening in that house.”

The 27-year-old double-amputee runner, whose stature peaked at the 2012 London Olympics and then plummeted when he shot model and television personality Reeva Steenkamp in the predawn hours of Valentine’s Day last year, stood in the dock writing in a pad and sometimes passing notes to defense lawyers. At one point, he smiled at a person sitting behind him. Steenkamp’s mother, June, sat near Pistorius but there was no communication between them.

The proceedings were broadcast on television, though Burger was not shown at her own request, and millions of people around the world followed a trial where the heady mix of a celebrity defendant and shocking allegations has drawn comparisons to the O.J. Simpson case two decades ago.

Prosecutors allege that Pistorius, who has been free on bail, shot Steenkamp after an argument. He has said he killed her after mistaking her for a nighttime intruder in his home, shooting her through the closed door of the toilet cubicle in his bathroom. Steenkamp, 29, was hit three times — in the head, elbow and hip area; a fourth bullet did not hit her.

Early testimony focused on whether the screams that Burger said she heard were those of a terrified woman about to be shot to death, as prosecutors allege, or were instead Pistorius’ desperate shrieks for help after a fatal mistake, as his defense lawyers contend.

Burger, who lives about 180 meters from Pistorius’ house, gave her account of the sequence of events in the pre-dawn hours of Feb. 14 last year.

“I was woken up by a woman’s petrified screams. I heard her screaming first,” Burger said. “Then I heard her call for help. Then I heard a man call for help three times. I then made a call. . . . I gave the phone to my husband and he spoke to security. Afterward, I heard the woman’s petrified screams again.”

Burger said she then heard four gunshots, with a gap between the first shot and the rest, and more screaming.

“I heard her voice during the shots,” she said. “Shortly after the shots was the last time I heard that woman.”

The chief defense lawyer, Barry Roux, opened his cross-examination by asking Burger if she thought Pistorius was a liar. She didn’t directly answer that, but questioned Pistorius’ version.

“I can only tell the court what I heard that evening,” Burger said. “I cannot understand how I could clearly hear a woman scream but Mr. Pistorius could not hear it.”

Roux, in an attempt to discredit the idea that Pistorius and Steenkamp had an argument before the shooting, contended that Burger heard just Pistorius screaming for help. He also suggested that she had not heard gunshots, but instead had heard the sound of the athlete breaking down the toilet door with a cricket bat after realizing he had shot Steenkamp.

“Could there have been shots fired when you were still asleep and you heard the screams afterward?” he asked. Roux’s constant challenges to Burger’s account of the sequence of events made for a contentious back-and-forth at times.

“Shall I repeat my question? It can’t be that difficult,” Roux said at one point, implying that the witness was evasive.

On another occasion, Burger said, “I didn’t sit there with a stopwatch and take down the timing of each shot.”

Pistorius pleaded not guilty to charges of murder and three other counts relating to shooting guns in public in unrelated incidents and illegal possession of ammunition.

Defense lawyer Kenny Oldwadge read a statement from Pistorius in which he said the killing was an accident and that there were inconsistencies in the state’s case.

Pistorius said he brought two fans in from the balcony during the night after speaking to his girlfriend in bed beside him. He said Steenkamp must have gone into the bathroom while he was getting the fans. Pistorius said he did not notice that she had gone, then heard the bathroom window open.

“I approached the bathroom, armed with my firearm, so as to defend Reeva and I,” Pistorius said in the statement. He said he then heard a noise in the toilet cubicle and was in a “fearful state” because he was unable to run away or defend himself physically since he was not wearing his prosthetic legs. He said he shouted at what he thought was an intruder and then shot through the toilet door, only later realizing that he had shot Steenkamp.

South Africa has a high crime rate and many people worry about daily security, but the prosecution case is likely to focus partly on Pistorius’ alleged character flaws and the suggestion that he was a hotheaded gun enthusiast.

If convicted on the murder charge, Pistorius could be sent to prison for at least 25 years before the chance of parole, the minimum time someone must serve if given a life sentence in South Africa. There is no death penalty. Analysts say he could be vulnerable to a lesser homicide charge.

Judge Thokozile Masipa will ultimately deliver the verdict and decide on any sentence. South Africa has no trial by jury. Masipa has 15 years’ experience as a judge, and was a reporter with a South African newspaper before she turned to law.

As Pistorius left the courthouse, people erupted in whoops and a few boos.